In order to transmit a voice signal in a limited frequency band in a voice transmission system, in general, the frequency band of the voice signal is narrowed and the band-narrowed voice signal is transmitted. Thus, a frequency band in which a voice reproduced by a receiver that has received the voice signal is included becomes narrower than the frequency band in which the original voice is included, resulting in deterioration of the quality of the voice reproduced by the receiver. For that reason, a technique that improves the quality of a reproduced voice by expanding a frequency band, in which a voice signal is included, in a pseudo manner is disclosed, for example, in Japanese Laid-open Patent Publication No. H8-248997.
In the technique disclosed in Japanese Laid-open Patent Publication No. H8-248997, spectrum envelope information and a residual signal are extracted from an input signal. Then, the frequency band of the spectrum envelope information and the frequency band of the residual signal are expanded, and a voice is synthesized by using the spectrum envelope information and the residual signal the frequency bands of both of which have been expanded.